Nickel-chromium alloys adapted for producing weldable sheet

ABSTRACT

NICKEL-BASE ALLOYS CONTAINING CHROMIUM, COBALT, MOLYBDENUM CARBON, HAFNIUM AND CORRELATED AMOUNTS OF ALUMINUM, TITANIUM, NIOBIUM, TANTALUM, AND VANADIUM AND ALSO BORON WITH OR WITHOUT ZIRCONIUM ARE PARTICULARLY ADAPTED FOR APPLICATIONS REQUIRING WELDED SHEET.

United States Patent 3,625,678 NICKEL-CHROMIUM ALLOYS ADAPTED FOR PRODUCING WELDABLE SHEET Edward Gordon Richards, West Hagley, Peter Lindsay Twigg, Halesowen, and Alfred John Fletcher, Hollywood, near Birmingham, England, assignors to The International Nickel Company, Inc., New York, NY. No Drawing. Filed May 19, 1969, Ser. No. 825,924 Claims priority, application Great Britain, May 21, 1968, 24,266/68 Int. Cl. C22c 19/00 U.S. Cl. 75-171 10 Claims ABSTRACT OF THE DISCLOSURE Nickel-base alloys containing chromium, cobalt, molybdenum carbon, hafnium and correlated amounts of aluminum, titanium, niobium, tantalum and vanadium and also boron with or without zirconium are particularly adapted for applications requiring welded sheet.

As will be appreciated by those skilled in the metallurgical art, the intended ultimate end use of a metal product normally dictates the minimum mechanical characteristics which the metal must offer for it to satisfactorily perform in use. And equally appreciated is the fact that such characteristics, inter alia, are usually an inherent feature of composition. However, predicting the expected behavior of a material based upon composition can be a most unrewarding experience. For example, the compositions of two alloys can be strikingly similar, yet their respective properties may be so dilferent as to render either one totally inapposite for uses fundamentally characteristic of the other. This is illustrated by reference to U.K. Pat. No. 1,075,216, in which the problem was to devise alloys capable of performing acceptably as rotor discs for gas turbine engines at temperatures on the order of about 600 C. or more. The alloys contained about 0.02% to 0.1% carbon, 10% to 17% chromium, 11% to 16% cobalt, to 9% molybdenum, 2.5% to 5.2% aluminum, up to approximately 1.7% titanium, 1.3% to 5% columbium, up to 2% vanadium, 0.001% to 0.01% boron, 0.01% to 0.1% zirconium, the balance essentially nickel.

Now, while such alloy compositions might be deemed similar to those contemplated and later described herein, they would be ill suited as weldable sheet alloys of high strength characteristics to which the present invention is primarily addressed. To explainrotor discs for gas turbines though they must manifest high stress rupture strength at elevated temperatures do not need to be cold worked or welded. Accordingly, these latter characteristics would be, relatively speaking, of minor significance. However, alloys in sheet form usually must be cold worked, and for fabrication into final products are commonly welded This requires alloys capable of resisting the propensity to crack either as a result of cold working or as a consequence of welding. Thus, the subject'invention is at least twopronged in nature, it is directed to alloys which (1) must satisfactorily respond to cold working and welding, and (2) in so doing the alloys must also afford good strength characteristics at both room and elevated temperatures.

It has now been discovered that the foregoing objectives can be achieved with nickel-chromium-cobalt-molybdenum alloys containing controlled amounts of aluminum with or without titanium, columbium, tantalum, vanadium, boron, zirconium and hafnium.

It is, an object of the present invention to provide nickel-chromium alloys which display good tensile and stress rupture strength at temperatures on the order of 750 C. or higher together with good cold formability and welding characteristics.

Generally speaking and in accordance with the present invention, alloys contemplated herein contain, in percentages by weight, about 0.02% to 0.12% carbon, about 10% to 17% chromium, about 11% to 16% cobalt, 5% to 9% molybdenum, aluminum, with or without titanium, from 3.5% to 4.5 the ratio of the aluminum to titanium being at least 2: 1, up to 3 niobium, up to 6% tantalum, up to 2% vanadium, with the provisos that (a) the sum of the niobium, and /2 the tantalum is from 0.7% to 3%, (b) the aluminum, titanium, vanadium, niobium and /2 the tantalium is from 5.5% to not more than 6.3% and (c) the ratio of the vanadium to columbium plus V2 the tantalium does not exceed 1.5 :1, about 0.001% to 0.007% boron, up to 0.08% zirconium, the percentage of zirconium plus 10 times the percentage of boron being at least 0.04%, about 0.01% to 0.05% hafnium, up to about 0.1% magnesium and the balance essentially nickel.

The role and effect of carbon, chromium, cobalt and molybdenum in respect of the subject alloys are much the same as in the alloys of the aforementioned U.K. Pat. No. 1,075,216. Thus, at least 0.02% carbon is required for adequate ductility and workability, but amounts above 0.12% reduce tensile and stress-rupture strength. A carbon range of 0.07% to about 0.1%, e.g. 0.09%, is quite suitable. With less than about 10% chromium, resistance to oxidation, particularly to attack by the products of combustion of gas turbine fuels, undesirably falls off. if it exceeds 17%, the alloys become prone to embrittlement on prolonged exposure to high temperatures. Preferably, the chromium is at least 12% most advantageously 14%, but does not exceed 16%. Cobalt exerts a marked influence on strength, high temperature tensile strength being reduced with cobalt contents below 11% or in excess of 16%. Accordingly, the percentage of cobalt is beneficially maintained from 13 to 15%. At least 5% molybdenum is desirable in respect of both tensile strength and ductility at high temperatures, but density and hot working are adversely affected and there is a greater tendency toward embrittlement on prolonged exposure to high temperatures when molybdenum is to the excess; therefore, it should not exceed 9%. Tungsten is not an equivalent of molybdenum since it impairs, for example, tensile ductility, and while the alloys are preferably tungsten free, up to 2% can be tolerated as a replacement for half its weight of molybdenum.

The elements aluminum, titanium, niobium, tantalum and vanadium are primarily responsible for the high strength characteristics of the alloys at elevated temperatures; however, only by controlling the total amount of the constituents and by balancing their respective percentages with one another is it possible to achieve desired strength levels without detrimental loss of workability, ductility and weldability. This, it has been found, requires that these elements be correlated such that the total sum thereof as represened by the following relationship,

percent Al-i-percent Ti-i-percent v +percent Nb+0.5 (percent Ta) is at least 5.5%; otherwise, tensile and stress-rupture strength are inadequate. Increasing this total boosts strength but at the expense of high-temperature ductility, weldability and hot workability; therefore, the sum total must not exceed 6.3% and advantageously is not more than 6.1%.

An aluminum plus titanium content of at least 3.5% is needed for adequate high temperature strength, but should this total much exceed 4.5% hot workability and high temperature ductility are greatly reduced. T00, the aforementioned aluminum to titanium ratio of at least 2:1,

is a prerequisite since at lower ratios high temperature tensile strength falls off sharply.

Niobium and tantalum are particularly effective in increasing the strength of the alloys in short time tests. For this purpose their combined content, as measured by the sum (percent Nb)+0.5 (percent Ta), must be at least 0.7%, but if this total exceeds 3%, so that the proportion of niobium and tantalum in the total content of hardener elements is excessive, the strength of the alloys in long time tests is lower than with a higher proportion of titanium and aluminum. Tensile ductility and hot workability of alloys containing vanadium as well as niobium or tantalum are better than those with the same total percentage of niobium or tantalum and preferably the alloys contain at least 0.5% vanadium. But since vanadium detracts from high temperature tensile and creep strengths and reduces oxidation resistance oxidation resistance it should not exceed 2% and the ratio of vanadium to (Nb-+0.5 Ta) should not exceed 1.5 :1.

Boron and zirconium enhance tensile ductility at temperatures of 600 C. and above, e.g., 750 C. The alloys should contain at least 0.001% boron, and preferably zirconium is also preesnt in an amount of at least 0.01%. In any event the total of (percent Zr)+10(percent B) must be not less than 0.04%. However, if more than 0.007% boron or 0.08% zirconium is present the weldability of the alloys is impaired. (All boron and zirconium contents in this specification were determined spectographically.)

Hafnium is essential to ensure that the alloys can be welded in sheet form. For this purpose a hafnium content of 0.01 to 0.05% is required, and advantageously from 0.02 to 0.04% is present.

Alloys in accordance herewith can be air melted, but in striving for the best creep properties and workability they are preferably melted and cast under vacuum. If melted in air, they are preferably refined by holding under vacuum in the molten state for some time before casting, the pressure being not more than 0.1 mm. Hg, preferably lower, e.g. 5 microns or less, with the temperature being suitably from 1400 C. to 1600 C. A holding time of at least 5 minutes, preferably at least minutes, is satisfactory. The presence of traces of magnesium in the alloys is found to improve workability, and however made they preferably contain at least 0.01% but not more than 0.1% magnesium. Thus, if the alloys are air melted they should be deoxidised with magnesium and if vacuum melted an addition of magnesium should be made so that at least the residual amount of 0.01% is present. Preferably the magnesium content is from 0.01 to 0.03%.

An alloy within the following compositional range affords excellent results and is quite advantageous: 0.07% to 0.09% or 0.1% carbon, 14% to 16% chromium, 13% to cobalt, 7% to 9% molybdenum, 2.9% to 3.5% aluminum, 0.5% to 1% titanium, the aluminum plus titanium being 3.5% to 4.5%, 0.7% to 2% niobium (including tantalum as an impurity), 0.5 to 1% vanadium, the sum total of the aluminum, titanium, niobium and vanadium being from 5.5% to 6.1%, 0.002% to 0.006% boron, 0.02% to 0.07% zirconium, 0.01% or 0.2% to 0.04% hafnium, 0.01% to 0.03% magnesium, and the balance essentially nickel.

Generally speaking plain sheet samples of alloys having such compositions exhibit at 760 C. a 0.2% tensile proof stress of at least 75 kg. f./mm. an ultimate tensile stress of at least 95 kg. f./mm. a life to rupture of at least 100 hours under a stress of 39.4 kg. f./mm. and a creep strain not exceeding 0.2% in 100 hours at 31.5 kg. f./mm.

The alloys being age hardenable require suitable heat treatment to develope the required combination of properties, both solution heating and aging treatments being necessary. High solution heating temperatures offer the highest in terms of creep resistance, while lower temperatures favor proof strength and tensile ductility. A suitable solution treatment comprises 10 minutes to 1 hour at 4 1050 C. to 1150 C., followed by rapid cooling at any practical rate, e.g. quenching in oil or water or even air cooling. Aging treatments can be carried out at temperatures in the range of 650 C. to 900 C., and may comprise one or more stages of heating for from 4 to 40 hours at successively lower temperatures in this range. A most beneficial overall heat treatment in reaching for the highest level of ductility together with good creep and tensile strength consists in solution heating for from 10 minutes to 1 hour at 1100 C. followed by aging from 4 to 20 hours at 750 C. to 900 C., preferably 850 C.

The manufacture of articles and parts from sheet generally involves cold working and welding operations. If left in the condition resulting from these operations, the properties of the material would be inadequate, and before it is put into service the article or part has to be heat treated to relieve internal stresses and to develop the optimum microstructure. Most high strength sheet alloys are prone to crack during this heat treatment, particularly when large amounts of cold work have been performed or heavily restrained welds have been made. It has been found in accordance herewith that the greatest resistance to cracking results when the alloy is solution heated at about 1100 C. and then aged at about 850 C. Moreover, the use of the higher aging temperature of 850 C. has been found to improve the tensile ductility, particularly after Welding, at the expense of a slight loss of strength.

For the purpose of giving those skilled in the art a better appreciation of the invention, the following illustrative data are given.

By way of example, three alloys were prepared by vacuum-melting having the analysed compositions, by weight, set forth in Table I. Each of the cast ingots was formed to sheet 1.2 mm. thick by forging followed by hot and cold rolling, and specimens cut from the sheet were subjected to tensile tests at several temperatures. Similar tests were performed on specimens containing a weld made by the tungsten-arc inert gas process, the test being performed with the weld transverse to the applied stress. All the specimens were tested after a heat-treatment comprising solution-heating for one-half to one hour at 1130 C. for Alloys Nos. 1 and 3 and 1150 C. for Alloy No. 2, air-cooling, and ageing for 1 6 hours at 750 C., the welded specimens being fully heat-treated after welding. The results are set forth in Table II. The improvement in ductility brought about by solution-heating at a lower temperature and ageing at a higher temperature is shown by the results of similar tests in Table III, which were carried out on specimens of Alloy No. 2 that had been solution-heated for 30 minutes at 1100 C., aircooled, aged for 8 hours at 850 C.

Further specimens cut from the sheet of Alloy No. 1 and heat-treated in the same way as the other specimens of this alloy were subjected to stress-rupture and creep tests with the results set forth in Table IV.

The importance of using amounts of boron, zirconium and hafnium in accordance with the invention to secure the desired combination of tensile properties and weldability is shown by the results of tests on a series of alloys in which the contents of these elements were varied but otherwise having the same nominal composition in accordance with the invention. Table V sets forth the results of tensile and stress-rupture tests on plain sheet specimens 1.2 mm. thick of the alloys that had been heat-treated by solution-heating at 1100 C. for 20 minutes, air-cooling, ageing for 16 hours at 750 C. and again air-cooling.

Alloys Nos. 4 and 6 were weldable under standard restrained conditions, but Alloy N0. 5, which contained boron and zirconium but no hafnium, was not. The addition of hafnium much improved the weldability and in conjunction with the specified levels of boron and zirconium much improved the tensile and stress-rupture strength.

,As will be understood by those skilled in the art, the term balance or balance essentially in referring to the nickel content does not exclude the presence of small amounts of other elements, commonly present as incidental elements, e.g., deoxidizing and cleansing constituents, and impurtiesv ordinarily associated therewith in small amounts which do not adversely affect the basic characteristics of the alloys. However, iron is undesirable since 7 it increases the tendency to embrittle n prolonged exposure at high temperatures, and it should he therefore not exceed 1%, though greater amounts up to may be tolerated if the percentage of chromium, molybdenum,

titanium, aluminum and niobium are all at the lower limits of their specified ranges. The other major impurities usually found are silicon and manganese, and not more than 1% of each should be present. Preferably the total content of all impurities and residual deoxidants does not exceed 2% TAB LE II Elong. on Test mm Alloy temp. 0.2% P.S. U.T.S.=, gauge No. C.) Specimen kg. f./mm. kg. t./mm. (percent) 1 Room {Plain 85.0 117. 0 14.0 temp. Welded 92.9 120. 5 10. 5 1 650 Plain 84.3 108.8 7.0 1 760 {Plain 81.1 102. 9 7. 5 Welded 80. 7 89. 3 2. 0 1 850 Plain 57. 8 79. 8 12. 5 2 760 {Plain 78. 7 101.0 =12 Welded- 75. 7 96. 5 =0 3 760 "{Plain 81. 6 102.0 5 Welded 83. 8 102. 2 4. 5

P.S.=Proof stress. U.T.S.=Ultimate tensile stress. 14.0 on 12 mm. gauge. 8.0 on 12 mm. gauge. 25 mm. gauge 4.0 on 12 mm. gauge.

TABLE III Elong. on Test 25 mm Alloy temp. 0.2% P.S., U.T.S gauge N0. C.) Specimen kg. f./rmn. kg. f./mm 2 (percent) 2 760 Plain 77. 1 103. 8 18 2 760 Welded 76. 1 97.5 15

TAB LE IV Stress-rupture properties Creep properties,

Test conditions Plain Welded time (hours) to- Stress Temp. Life Elong. Life Elong. 0. 1% 0. 2% (kg. f./mm. 0.) (hrs.) (percent) (hrs.) (percent) strain strain TABLE V Stress-rupture properties at 39.4 Tensile props. at 760 0. kg. iJmm.

B Zr Hf 0.2% P.S., U.T.S., -Elong. Temp. Life Alloy No. (percent) (percent) (percent) kg. f./mm. kg. fJrnm. (percent) C.) (hrs.)

Although the present invention has been described in We claim:

conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

1. An alloy possessing good cold workability and tensile ductility characteristics in combination with good tensile and stress rupture strength properties at temperatures on the order of 750 C. or higher, said alloy consisting of about 0.02% to 0.12% carbon, about 10% to 17% chromium, about 11% to 16% cobalt, about 5% to 9% molybdenum, about 3.5% to 4.5% of aluminum with or manganese and less than 1% iron.

Without titanium, the ratio of the aluminum to any titanium being at least about 2:1, up to 3% niobium, up to 6% tantalum, up to 2% vanadium with the provisos that (a) the sum of the niobium and half the tantalum is from 0.7% to 3%, (b) the total sum of the aluminum, titanium, niobium, /2 the tantalum and the vanadium is from 5.5% to not more than 6.3% and (c) the ratio of any vanadium to the sum of any niobium plus /2 of any tantalum does not exceed about 1.5:1, about 0.001% to 0.007% boron, up to 0.08% zirconium, the zirconium plus 10 times the boron being at least 0.04%, about 0.01% to 0.05% hafnium, up to less than 0.1% magnesium, and the balance essentially nickel.

2. An alloy in accordance with claim 1 in the form of sheet.

3. An alloy in accordance with claim 1 in which the alloy forms at least one component of a welded structure.

4. An alloy sheet in accordance with claim 2 in which it comprises at least one component of a welded structure.

5. An alloy in accordance with claim 1 which contains magnesium in an amount up to 0.03%

6. An alloy in accordance with claim which contains at least about 0.01 magnesium.

7. An alloy in accordance with claim 1 consisting of about 0.07% to 0.09% carbon, about 14% to 16% chromium, about 13% to 15% cobalt, about 7% to 9% molybdenum, about 2.9% to 3.5% aluminum, about 0.5% to 1% titanium, 0.7% to 2% niobium, 0.5% to 1.5% vanadium, the sum of the aluminum, titanium, niobium and vanadium being 5.5 to not more than 6.1%, 0.002% to 0.006% boron, 0.02% to 0.07% zirconium, 0.01% to 0.04% hafnium, 0.01% to 0.03% magnesium, the balance being essentially nickel.

8. An alloy in accordance with claim 7 in the form of sheet.

9. An alloy in accordance with claim 7 in which the alloy forms at least one component of a welded structure.

10. An alloy sheet in accordance with claim 8 in which it comprises at least one component of a welded structure.

References Cited UNITED STATES PATENTS 3,479,157 11/1969 Richards et al. 171

RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 148-325 

